1. Field of Invention
The techniques and apparatus described herein relate to semiconductor structures and devices, and particularly to aluminum nitride based semiconductor structures and devices.
2. Discussion of the Related Art
Improved power transistors are desired for advanced transportation systems, more robust energy delivery networks and new approaches to high-efficiency electricity generation and conversion. Such systems rely on very efficient converters to step-up or step-down electric voltages, and use power transistors capable of blocking large voltages.
Nitride semiconductor devices have been proposed as offering the potential for producing high-efficiency power electronics demanding high blocking voltages and low on-resistances. One nitride-based semiconductor material that has been investigated is gallium nitride (GaN). A GaN-based HEMT (High Electron Mobility Transistor) can be formed by confining carriers in a two-dimensional electron gas (2DEG). Carrier confinement is conventionally achieved by forming a material with a higher bandgap on a material of lower bandgap. GaN-based HEMT devices have been formed using a layer of GaN with an overlying layer of aluminum gallium nitride (AlGaN) of a higher bandgap than GaN.
FIG. 1 shows an example of a conventional HEMT 10 formed of GaN and AlGaN semiconductor layers 2, 4, respectively. As shown in FIG. 1, the HEMT 10 is formed on a substrate 6 of a material such as silicon carbide (SiC), silicon (Si) or sapphire, for example. Due to the lattice mismatch between the substrate and a III-V semiconductor material such as GaN, on top of the substrate is formed a buffer layer 8 typically having a graded composition such that the lattice constant changes gradually from its lower surface to its upper surface. At its upper surface, the buffer layer 8 has a higher lattice constant than at its lower surface to accommodate the lattice constant of GaN. As shown in FIG. 1, a GaN layer 2 is formed on the buffer layer 8. A layer of AlGaN 4 is formed on top of the GaN layer 2. Since the bandgap of AlGaN is higher than that of GaN, a two-dimensional electron gas 9 is formed in the GaN layer 2 at the interface between the GaN and AlGaN layers 2, 4. Source and drain contacts S, D as well as a gate G and passivation layer 11 are formed over the AlGaN layers. In operation, a voltage applied to the gate modulates the two-dimensional electron gas 9, which acts as a channel region of the transistor.